As is described in U.S. Pat. No. 4,610,946, aluminium with a purity of 99.50% by weight or higher and having small amounts of iron and silicon therein are known to produce good quality printing plates using the processing steps described in that patent. As the patentee points out, however, existing alloys such as AA1050 are known to be adversely affected by the heating step needed to "burn in" or "stove" the finished lithographic printing plate in order to harden the image areas and thereby enhance the printing plate's life. Stoving can reduce the strength and cause distortion of lithographic sheet material by causing recovery or recrystallisation of the heavily cold worked metal. A useful indication of the likely amount of distortion that may occur is provided by measuring the change in yield or ultimate tensile strength caused by stoving. A large loss in strength indicates an unacceptable level of distortion, and difficulties in handling and mounting for use in service.
U.S. Pat. No. 4,610,946 seeks to provide an improved printing plate capable of withstanding temperatures higher than 280.degree. C. without any distortion of the plate which could lead to difficulties in mounting the printing plate on the printing cylinder of a press. The improvement described in that patent consists of the addition of from 0.02 to 0.20% by weight of zirconium, whilst maintaining the remaining alloying elements and impurities at the levels set for the known alloy AA1050. Of these alloying elements the following preferred ranges are described in U.S. Pat. No. 4,610,946:
______________________________________ Iron 0.40% by weight or less Silicon 0.20% by weight or less Manganese 0.05% by weight or less Copper 0.05% by weight or less Titanium 0.05% by weight or less Residual Impurities 0.05% by weight or less ______________________________________
As can be seen from the results given in Tables 2, 3 and 4 of U.S. Pat. No. 4,610,946, at temperatures below 280.degree. C. the improvement in yield strength and performance of the finished printing plates were only very slightly improved by the addition of zirconium, and further that there was little difference between addition at the rate of 0.05% Zr and of 0.16% Zr.
In order to avoid the distortion problems associated with stoving temperatures greater than 280.degree. C., temperatures of about 240.degree. C. are often preferred by those in the art, but even at these temperatures some softening of the aluminium base alloy is noted, which softening adversely affects the quality of the printing plate for the reasons previously mentioned.
In an attempt to overcome this softening, an addition of zirconium was tried by the Applicants in line with the teaching of U.S. Pat. No. 4,610,946, but although a small improvement was noted, some softening was still evident. In addition, it was found that batch interannealing of metal with up to 0.13% zirconium resulted in the growth of extremely large grains which are elongated in the rolling direction, some of which were several centimetres long. Such grains were found to adversely affect the quality of the surface of the electro-grained sheet, although the Applicants noted that these large grains did not form if continuous annealing was used.
In EP-A-0289844, the problem of the softening of aluminium printing plates at about 240.degree. C. is recognised and the described solution consists of closely limiting both the manganese and iron content of the aluminium alloy and of utilizing a fabrication process which is difficult to control in order to achieve a particular structure in the alloy in which the type, quantity and number density of the secondary phases therein are carefully controlled. As will be appreciated by those skilled in the art careful control of the processing conditions of sheet material is difficult and expensive to achieve on a commercial scale, and in any event it has been found by the Applicants that the improvement in resistance to this softening of the alloy is not as good as is desired.
There therefore exists a need for an aluminium alloy which resists more successfully the softening encountered during heating at about 240.degree. C. without having a utilize the type of difficult and expensive production conditions described in EP-A-0289844, and which is also amenable to batch annealing as well as continuous annealing.